I know of a local reptile shop that keeps their cages that high, they more or less keep their shop the same as the outdoor humidity level. I live in Orlando, Fl where the outdoor humidity % was 91% today. Of course things like heat lamps will reduce the humidity level in their cages and during the winter (if you consider 60 degrees winter lol) the humidity is marginally lower as well.
As far as Africa is concerned... you also have to consider that BP's will stay in an underground burrow during the day. Even if the ambient humidity level drops down below 60% (which I'm sure that it does) their burrows should stay more humid.